The first major processing step in a petroleum refinery is typically the distillation of crude oil into fractions corresponding to various boiling point ranges. By way of example, a typical distillation produces a vapor (light hydrocarbon) fraction, a light straight run gasoline fraction, a naphtha fraction, a diesel base fraction and a gas oil fraction. The distillation residuum is referred to as the topped crude.
Prior to distillation the crude oil is usually subjected to a desalting operation. The desalting operation is typically followed by a preheating operation to heat the crude oil to a temperature sufficient to effect vaporization of a portion of the crude oil. The preheated feed is introduced into a distillation means such as, for example, a fractionation column. The distillation separates the crude oil into the desired fractions which serve as feedstocks for various other operations at the refinery.
The amount of heat supplied to the crude oil in the preheating means determines (when all other conditions are held constant) the extent to which the crude oil is vaporized. It is only desired to vaporize the components corresponding to the distilled fractions. It is not desired, at this stage, to vaporize the portion of the crude oil corresponding to the distillation residuum. If too much heat is supplied to the crude oil too much vapor is produced. The excess vapor must be condensed in the fractionation column before it can be removed as residuum (i.e. topped crude). As a result of the overheating, energy is needlessly wasted. If too little heat is supplied to the crude oil, too little vapor is produced. As a result certain components desired in the fractions are not vaporized and are lost to the residuum. Consequently, it is desirable to control the preheating of crude oil such that vaporization of the crude oil occurs to the extent desired.
Given this description of the problem it is readily apparent that it would also be desirable to effect a similar control over the preheating of distillation feeds other than crude oil. Accordingly my invention, which provides a solution to the above-identified problem, is broadly applicable to distillation operations in general, even though it is especially well suited to crude oil distillation.